Deschutes County appears poised to rezone a nearly 500-acre former pumice mine west of Tumalo, clearing the way for future housing developments.

The Tumalo Irrigation District owns the land and last year sought to change its zoning to allow single-family homes or small farms on the reclaimed mine. A hearings officer hired by the county denied that request in February, ruling the irrigation district didn’t do enough to prove the rezoning wouldn’t result in a “significant impact” on roads. The irrigation district appealed.

But during its Wednesday meeting, the Deschutes County Commission asked county staff to draft a formal rejection of the hearings officer’s decision that commission members will vote on later this fall. Commissioners told Adam Smith, the county’s assistant legal counsel, to include a requirement that the landowners address concerns about transportation and sinkholes.

“I’m confident that we could come up with some kind of condition of approval that will ensure that this issue is a belt-and-suspenders approach and it will be addressed at a later date,” Smith said. “If and when development occurs, there will be additional regulatory provisions that will come into play that will probably doubly assure that some of these things will be addressed.”

Tumalo Irrigation District doesn’t have a plan for development, but it anticipates the property will eventually be used for single-family homes, hobby farms and a recharge reservoir for irrigation piping projects. Because the new zone requires lots to be a minimum of 10 acres, it would be limited to 54 homes.

But the county also allows cluster developments — which permit rural homes to be on smaller lots and close to each other — if developers apply for a conditional use permit and meet additional regulations. A set of cluster developments on the property could result in up to 74 homes.

The county’s hearings officer ruled the irrigation district should have considered those 74 possible homes when analyzing effects on transportation. County commissioners disagreed with the hearings officer, in part because it could set a costly precedent for future proposed developments.

“This is not the first time it has come before us that some would want us to use conditional uses as well as outright (uses),” Commissioner Tammy Baney said.

Central Oregon LandWatch, a nonprofit environmental watchdog that opposes the rezoning, agreed with the hearings officer. LandWatch cited a history of erosion and sinkholes on the property, particularly along Bill Martin Road, which runs through the property before dead-ending just south of U.S. Highway 20.

The road was also the subject to a court case, dismissed on appeal earlier this month, filed by several neighboring property owners who allege the irrigation district failed to properly maintain the road.

LandWatch used that lawsuit to argue that Tumalo Irrigation District violated county land use regulations. It attempted to block the zoning change because county rules prevent commissioners from approving land use applications for properties that aren’t in compliance with regulations.

Outside lawsuits or allegations that landowners are breaking rules shouldn’t be enough to derail an application, Baney said.

Commissioners came closer to agreeing with LandWatch on the issue of whether the former pumice mine has been restored to a point where it’s suitable for development. LandWatch argued that the county should evaluate sinkhole risks from improper filling before moving forward with rezoning, but commissioners decided that they would leave that level of scrutiny for when developers come with plans to build on the site.

Commissioner Tony DeBone said the reclaimed land will settle.

“Any actively mined site that has been reclaimed is not going to have an open void in the ground,” he said. “It’s going to have some dirt, and maybe that dirt will settle. It won’t be a lava tube. It will be a settling situation.”

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